This invention relates to a novel para- or meta-terphenyltetracarboxylic acid, a dianhydride thereof and a process for preparing the same.
As an aromatic tetracarboxylic acid or a dianhydride thereof, pyromellitic acid or a dianhydride thereof, benzophenonetetracarboxylic acid dianhydride, etc. are known predominantly as a raw material for polyimide resins.
On the other hand, as a terphenyltetracarboxylic acid and para-terphenyl-2,3,2",3"-tetracarboxylic acid 2,3:2",3"-dianhydride of the formula (I): ##STR1## is known as described in Offenlegungsschrift (West Germany) No. 21 00 391, the Specification of British Patent No. 1,338,932 and the Specification of U.S. Pat. No. 3,891,633.
The tetracarboxylic acid dianhydride of the formula (I) is a compound which can be synthesized through the route illustrated by the following reaction equation (II): ##STR2##
Namely, in the above reaction, an intermediate compound is obtained through a Diels-Alder reaction between one molecule of 1,4-bis(.alpha.-furano)benzene and two molecules of maleic anhydride, and then dehydrated with a concentrated sulfuric acid or a polyphosphoric acid to yield the tetracarboxylic acid dianhydride. Because of the nature of the reaction, the acid anhydride groups come to locate at 2,3- and 2",3"-positions.
The compound of the formula (I) is used as a crosslinking agent for epoxy resins or phenol-formaldehyde resins.
As meta-terphenyltetracarboxylic acid, there has been reported only one such acid in the Specification of French Patent No. 1,556,159 (Chemical Abastract, Vol. 71, 4960 k), and synthesis thereof can be illustrated by the following reaction equation (III): ##STR3## tetracarboxylic acid dianhydride+SO.sub.2 +HCl (III)
Namely, in the above reaction, meta-benzenedisulfonyl chloride is reacted with an excess phthalic anhydride under reflux of the latter with generation of sulfur dioxide gas (SO.sub.2) and hydrogen chloride gas using copper as a catalyst to yield meta-terphenyltetracarboxylic acid dianhydride. The resulting compound had a melting point m.p. of 130.degree. to 165.degree. C.
While polyimide resins to be obtained from a prior art aromatic tetracarboxylic acid dianhydride and a diamine compound, e.g. diaminodiphenyl ether, diaminodiphenylmethane, etc., are known as a heat-resistant resin, much higher heat resistance is desired with the recent advance of technologies.
Also, the para-terphenyltetracarboxylic acid dianhydride of the abaove formula (I) is known as a crosslinking agent for epoxy resins or phenol resins. However, said para-terphenyltetracarboxylic acid dianhydride has the acid anhydride groups at 2,3- and 2",3"-positions. Accordingly, the polyimide resin synthesized from said anhydride is difficult to have higher molecular weight, resulting in insufficient heat resistance.
Also, in the above meta-terphenyltetracarboxylic acid dianhydride, the positions of the acid anhydride groups are not definite because of the nature of the synthesis. Besides, in view of the melting point of the resulting compound ranging from as wide as 130.degree. to 165.degree. C., it can be understood that it is a mixture of compounds having acid anhydride groups at different positions. Use of such tetracarboxylic acid dianhydride as a raw material for polyimide resin will result in a polyimide resin of low molecular weight, and its heat resistance is insufficient.